This invention relates to weighing apparatus and more particularly to such apparatus for use in weighing vehicles in motion.
The ability to weigh vehicles in motion offers many advantages over static weighing and reliable, reasonably accurate systems for doing so have long been sought. A particularly advantageous use for such systems is to monitor truck traffic as the trucks travel along interstate highways. This has usually been accomplished by requiring trucks travelling on interstate highways to enter weigh stations where they are weighed using static scales. Truck operators may, however, go to considerable lengths to avoid a weigh station for various reasons. This avoidance reduces the amount of data available to regulatory authorities as to truck traffic as well as placing heavy trucks on roads not designed for such traffic.
Weigh-in-motion (WIM) systems offer the ability to monitor a larger number of sites, the ability to monitor truck traffic without truck operators being aware of it (thereby minimizing the scale avoidance problem), reduced cost and the provision of information as to the wheel loads being imposed on the roadway. Disadvantages of known WIM systems include relative inaccuracy in comparison with static scales and difficulties in installation and maintenance of the system. There are also problems associated with calibration of the system to yield static weights from the dynamic weight information.
A WIM system may include a pair of scales installed side-by-side in one lane of an interstate highway, one scale to weigh each wheel on the opposite sides of a truck as it moves over the scales. Alternatively, a single scale may occupy the entire lane to weigh the truck axle. In either case, a scale may typically include a base anchored in concrete beneath the surface of the roadway, a weighing platform level with the surface of the roadway and a number of load cells mounted between the platform and the base to provide a signal indicating the load applied by the wheel or axle passing over the platform.
It will be apparent that such a system is subject to a great deal of abuse, including loads tending to move the platform horizontally with respect to the base in the direction of the moving traffic and loads tending to move the platform vertically off the base. Vehicles moving at highway speeds exert significant suction on the roadway. This upward force is exerted upon the scale platform as the vehicle passes over the scale. A typical way to solve this problem is to make the mass of the platform very large. This solution, however, results in relatively slow system response for obtaining weight information.
Another solution is that the platform be anchored to the base by means of bolts. The bolts cannot, however, be excessively tight and must allow for relative vertical movement between the platform and the base to permit the load cells to flex under load. Such play in the bolts also permits a certain amount of relative horizontal movement between platform and base. Another disadvantage is lack of firm constraint in the vertical direction against forces tending to lift the platform off the base. The "check bolt" arrangements thus allow platform motion with consequent high impact and shock forces. Over time, this may result in shearing of the bolts, much wear and tear on the installation, reduced weighing accuracy and a greater need for regular maintenance.